Recently the availability of strategic elements such as chromium, columbium, tantalum, and cobalt has been reduced for a variety of economic reasons. The reduced availability of these strategic elements is of great concern to the aerospace industry as these elements are essential to the production of modern aircraft gas turbine engines. The threat of loss of supply of these elements has necessitated consideration of alternatives to existing nickel-base superalloys which generally include substantial additions of strategic elements.
One such alternative is iron-base alloys and, specifically, iron-aluminium alloys. Iron aluminides offer great potential for high corrosion and oxidation resistance, low cost, low weight, and low strategic metal content. Of particular interest are the very good density-compensated mechanical properties. However, previous research on iron-15% aluminum (16 Alfenol), iron-16% aluminum-3.5% molybdenum-0.1% lanthanum (Thermenol) and iron-8/14% aluminum-3% titanium alloys has indicated that alloys based on the iron-aluminum system lack ductility to such a degree that they can neither be fabricated satisfactorily nor utilized because of brittleness. These percentages, as well as those elsewhere used in this description, are in terms of weight percent.
In general, at least 8% aluminum content is required for adequate oxidation resistance. At the time of earlier investigations in iron-aluminum systems, it was expected that over 5% aluminum content would embrittle the alloy. However, improvements at that time in melting and refining practice yielded lower oxygen contents and ductility was improved for iron-aluminum alloys with up to 12% aluminum content.
Ultimately, however, lack of ductility at low temperatures and poor hot workability discouraged investigators. Recently, several technologies including directionally solidified/single crystal castings and rapidly solidified powders have emerged which could improve the shortcomings of the iron-aluminum alloy system. Single crystal castings eliminate grain boundary contributions to poor ductility and increase rupture strength. Rapidly solidified powder (RSP) technology provides extended solid solubility limits, homogeneous microcrystalline structures, and greatly refined grain size which would be expected to improve low temperature ductility, but in many instances at some sacrifice to creep strength.
One currently accepted approach to the development of iron-aluminum alloys is to produce rapidly solidified powders of appropriate compositions which are consolidated by vacuum canning and extrusion. After thermal treatments, the alloys are analyzed for the properties of interest. This procedure is costly and time consuming and such goals could also be achieved by employing investment casting of a large number of experimental compositions for which the desired high temperature properties could be readily determined. Once promising compositions are identified, other desirable properties can be determined by processing a few selected compositions by powder metallurgy (P/M) techniques to demonstrate particularly the ability to achieve a ductile material at low temperature.
In the past, and currently, investigators have sought to develop iron-aluminum alloys for low strength, high temperature (1800.degree. F.) applications and for high strength, low temperature (600.degree. F.) applications. Applications such as high pressure compressor and low pressure turbine blade vanes and other components require adequate creep rupture properties at about 1200.degree. F. Traditionally low pressure turbine blades employ nickel-base or cobalt-base superalloys primarily because of the creep resistance requirements in the 1200.degree. F. range. Consequently, an iron-aluminum alloy composition having elevated temperature strength approaching conventional superalloys with adequate low temperature ductility would be desirable.
Accordingly, it is an object of the invention to provide an iron-aluminum alloy composition having excellent oxidation resistance and creep rupture properties as an alternative to conventional nickel-base and cobalt-base superalloys for elevated temperature applications.
Another objective of the invention is to provide a material for high temperature applications having a high density-compensated strength.
It is a further objective of the invention to provide an iron-aluminum alloy composition having strengths at 1200.degree. F. approximating the strengths of conventional heat resistant alloys such as A286 and X40.
A still further object of the invention is to provide an iron-aluminum alloy composition having suitable ductility at low temperatures.
Additional objects and advantages will be set forth in part in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention.